Resonant Column Test for Unsaturated Soils With Suction–Saturation Control

Comprehensive characterization of the small strain shear modulus Gmax of an unsaturated soil specimen during hydraulic hysteresis requires precise control of the stress state, matric suction, degree of saturation, as well as knowledge of the void ratio. This paper describes the details and typical results from a test method which incorporates the axis translation technique for suction control, a flow pump for degree of saturation control, and a vertically oriented proximeter to infer changes in void ratio into a fixed–free resonant column setup. A unique aspect of this test method is the operation of the flow pump to reach equilibrium points on the hysteretic soil–water retention curve (SWRC) for measurement of Gmax. The flow pump, which controls water flow to or from the soil specimen through a high air-entry ceramic disk, is guided using a feedback loop involving the matric suction measured at the boundary of the specimen. Values of Gmax were measured at different equilibrium points on the primary drainage path of the SWRC up to the point of water occlusion, then along a scanning imbibition path of the SWRC. The results indicate a greater shear modulus during imbibition, consistent with trends and magnitudes noted in the technical literature.